SMTP | Notion

SMTP is a protocol used to send emails over the internet. It can work between an email client and a mail server, or between two mail servers. It's often combined with IMAP or POP3 to receive emails, and while SMTP is mainly client-server, servers can also communicate with each other using it.
SMTP servers typically use port 25 for connections, but newer servers also use port 587 for better security and performance.

cat /etc/postfix/main.cf | grep -v "#" | sed -r "/^|s*$/d"

Command	Description
`AUTH PLAIN`	AUTH is a service extension used to authenticate the client.
`HELO`	The client logs in with its computer name and thus starts the session.
`MAIL FROM`	The client names the email sender.
`RCPT TO`	The client names the email recipient.
`DATA`	The client initiates the transmission of the email.
`RSET`	The client aborts the initiated transmission but keeps the connection between client and server.
`VRFY`	The client checks if a mailbox is available for message transfer.
`EXPN`	The client also checks if a mailbox is available for messaging with this command.
`NOOP`	The client requests a response from the server to prevent disconnection due to time-out.
`QUIT`	The client terminates the session.

You can use the telnet tool to connect to an SMTP server and start the session by typing the commands HELO or EHLO. telnet 10.129.14.128 25